iOS 9.3.6 (build 13G37) is an official Apple over-the-air update for the iPhone 4s, released on 2019-07-22T17:06:53Z. The update package is 856.15 MB. This is a delta (partial) update that requires 8.4 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 0234e93f0d99e69214aa468d3f261fa7097ce8ca.zip.